expose call function with empty default args as exec

lib/youtube-dl.js

@@ -62,6 +62,20 @@ var ytdl = module.exports = function(url, args, options) {
 };
 
 
+/**
+ * Calls youtube-dl with some arguments and the `callback`
+ * gets called with the output.
+ *
+ * @param {String} url
+ * @param {Array.<String>} args
+ * @param {Object} options
+ * @param {Function(!Error, String)} callback
+ */
+ytdl.exec = function (url, args, options, callback) {
+    return call(url, [], args, options, callback);
+}
+
+
 /**
  * Calls youtube-dl with some arguments and the `callback`
  * gets called with the output.